This line of thinking has always baffled me. A film's quality is judged entirely on its own merits, not the merits of other films it shares connections with. This is because a narrative needs to function and operate correctly within itself otherwise it fails on the most fundamental of levels, which is telling a story. Even films that are direct sequels to or within the same franchise as each other need to operate independently, such as Star Wars, The Lord of the Rings, The Dark Knight trilogy, even the X-Men films.

If the Avengers was character-less and just a special effects laden punch up then it would be a bad film period. Even if all the previous films had been universally adored. While it's true that you can get more from a film in a series if you're familiar with the other films, they can't influence the specific mechanisms and workings of any individual film's narrative. This cuts both ways in a sense as it explains why people can hate any of the previous Marvel films but enjoy the Avengers, such as Ebert loathing Thor but not the Avengers.

I'd say one of the Avengers' greatest strength, if not the greatest, is Whedon's wonderfully elegant and tightly written script. It uses the characters of other films and utilises them in a way that not only provides a greater experience for those familiar with them but also explains them to newcomers and propels its own internal narrative. On the most basic level, Whedon's film needs characters to work. Therefore he needs to introduce and characterise them, it doesn't matter how many previous appearances Iron Man or Thor have made, Whedon has to build them from scratch into his specific narrative. It seems ridiculous to me that people can say that the solo films 'did the work' of introducing/developing characters so the Avengers 'didn't have to' as that suggests that the Avengers could possibly have worked while completely lacking such fundamental building blocks of stories.

I very much enjoy First Class and I'd also say that the Avengers has just as much as 'meat' in terms of of its characters and storytelling. Maybe even more given the biggest weakness of First Class is the bloated nature of its script compared to the elegance of the Avengers.

Dude, you are taking me too literally, of course there is some character work in the film, but we do not get to see what makes these characters 'tick' in those particular ways, before we get to the interactions, we need the solo movies for that.

I said this already...

- We don't know what made Cap so special(in actual fact , we don't know why he was so earnest and clean cut in the first movie either, but for the purposes of what we are givem , I mean, we don't know of his weak, humble beginnings, it is an added thrill to know this when he goes up against a god)

- the dicotomy of Iron-Man's irreverence and heroics would be puzzling, and less interesting.

- Thor's speeches to Loki would carry less weight if we had not seen him grapple with his own problems when it came to ruling.

edit: and I'm sorry, but no, different rules apply when dealing with chapters in a series of films, you can't critisice chapter two if it brings no sense of what makes these characters 'tick'(as long as there is some further characterisation), but you can critisice chapter one if it does not do this job.